Massive stars exploding in a He-rich circumstellar medium XII. SN 2024acyl: A fast, linearly declining Type Ibn supernova with early flash-ionisation features

We present a photometric and spectroscopic analyses of the Type Ibn supernova (SN) 2024acyl. It rises to an absolute magnitude peak of M_o = −17.58 ± 0.15 mag in 10.6 days, and displays a rapid linear post-peak light-curve decline in all bands (e.g. γ_0 − 60(V) = 0.097 ± 0.002 mag day⁻¹), similar to most SNe Ibn. The optical pseudobolometric light curve peaks at (3.5 ± 0.8)×10⁴² erg s⁻¹, with a total radiated energy of (5.0 ± 0.4)×10⁴⁸ erg. The spectra are dominated by a blue continuum at early stages, with narrow P-Cygni He I lines and flash-ionisation emission lines of C III, N III, and He II. The P-Cygni He I features gradually evolve and become emission-dominated in late-time spectra. The Hα line is detected throughout the entire spectral evolution, which indicates that the circumstellar material (CSM) is helium-rich with some residual amount of hydrogen. Our multi-band light-curve modelling yields estimates of the ejecta mass of M_ej = 0.49^+0.11_−0.09 M_⊙ with a kinetic energy of E_k = 0.06^+0.01_−0.01 × 10⁵¹ erg, and a ⁵⁶Ni mass of M_Ni = 0.018 M_⊙. The inferred CSM properties are characterised by a mass of M_CSM = 0.51^+0.05_−0.04 M_⊙, an inner radius of R₀=17.8^+3.6_−3.0 AU, and a density of ρ_CSM = (8.3^+2.7_−1.2) × 10⁻¹² g cn⁻³. The multi-epoch spectra are well reproduced by the CMFGEN/ he4p0 model, corresponding to a He-ZAMS mass of 4 M_⊙ (H-ZAMS mass 18.11 M_⊙, pre-SN mass 3.16 M_⊙). These findings are consistent with a scenario of an SN powered by ejecta-CSM interaction originating from a low-mass helium star that evolved within an interacting binary system where the CSM with some residual hydrogen may originate from the mass-transfer process. We also discuss an extreme scenario involving the possible merger of a helium white dwarf. In addition, a channel of core-collapse explosion of a late-type Wolf-Rayet (WR) star with hydrogen, or a transitional star between an Of and a WR type (e.g. an Ofpe/WN9 star) with fallback accretion cannot be entirely ruled out.